Datalogging Explained: What to Log and What It Tells You
A datalog is the only honest account of what your engine did. Without one, a diagnosis of a tuning problem is a guess with confidence attached.
Log boost, ignition timing, knock correction, intake air temperature, lambda against target and injector duty in a single steady pull. The value is in comparing the same pull before and after a change.
Tuners ask for logs because the alternative is opinion. A good log takes a few minutes to record and answers questions that no amount of description will.
What to record
The core channels are engine speed, manifold pressure or boost, throttle position, ignition timing, any knock retard or correction counter, intake air temperature, coolant temperature, measured lambda alongside the target lambda, injector duty cycle and fuel trims. Battery voltage is worth adding since it affects injector dead time and coil charging. Log at the highest rate your interface supports, because slow sampling can miss a brief knock event entirely. If your car has an ethanol content sensor or a fuel pressure sensor, log those too, since fuel pressure dropping under load is a common and easily misread fault.
How to take a useful pull
Consistency is what makes logs comparable. Use a gear with a long enough ratio to load the engine properly, typically third or fourth, start from low rpm at steady throttle, then go to full throttle and hold it to your normal limit in one continuous pull without lifting or changing gear. Do it with the engine at full operating temperature and note the ambient conditions and the fuel in the tank. One clean pull is worth more than ten partial ones. Have a passenger operate the laptop or use a logger that starts on throttle position, because doing it yourself is unsafe and produces bad data anyway.
Reading what comes back
Ignition timing being pulled as rpm rises means the ECU is hearing knock and protecting the engine, and the usual causes are fuel quality, heat or too much boost. Boost that overshoots and then tapers points at wastegate control rather than at the turbo itself. Intake temperature climbing steeply through the pull is heat soak, which flatters the first run of the day and punishes the third. Measured lambda drifting lean of target while duty cycle rises toward 100 percent is a fuel supply limit. Each of those has a different fix, and the log is what separates them.
The conditions that matter
- ✓One clean third or fourth gear pull at operating temperature beats many partial logs.
- !Timing pulled under load means knock is being detected, not that the tune is safe.
- !Log before and after every change, or you cannot attribute the result to it.
Sources
These are the provisions the road-legality rules linked above rest on.
General guidance only. Details vary by exact vehicle and change over time; confirm with a reputable specialist before modifying.